<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>造型函数</title>
</head>
<body>
  <script src="../common/lib/gl-renderer.js"></script>
  <canvas width="512" height="512"></canvas>
  <script>
    const vertex = `
      attribute vec2 a_vertexPosition;
      attribute vec2 uv;

      varying vec2 vUv;

      void main() {
        gl_PointSize = 1.0;
        vUv = uv;
        gl_Position = vec4(a_vertexPosition, 1, 1);
      }
    `;

    const fragment = `
      #ifdef GL_ES
      precision highp float;
      #endif

      varying vec2 vUv;

      float line_distance(in vec2 st, in vec2 a, in vec2 b) {
        vec3 ab = vec3(b - a, 0);
        vec3 p = vec3(st - a, 0);
        float l = length(ab);
        return cross(p, normalize(ab)).z;
      }

      float seg_distance(in vec2 st, in vec2 a, in vec2 b) {
        vec3 ab = vec3(b - a, 0);
        vec3 p = vec3(st - a, 0);
        float l = length(ab);
        float d = abs(cross(p, normalize(ab)).z);
        float proj = dot(p, ab) / l;
        if(proj >= 0.0 && proj <= l) return d;
        return min(distance(st, a), distance(st, b));
      }

      float triangle_distance(in vec2 st, in vec2 a, in vec2 b, in vec2 c) {
        float d1 = line_distance(st, a, b);
        float d2 = line_distance(st, b, c);
        float d3 = line_distance(st, c, a);

        if(d1 >= 0.0 && d2 >= 0.0 && d3 >= 0.0 || d1 <= 0.0 && d2 <= 0.0 && d3 <= 0.0) {
          return -min(abs(d1), min(abs(d2), abs(d3))); // 内部距离为负
        }
        
        return min(seg_distance(st, a, b), min(seg_distance(st, b, c), seg_distance(st, c, a))); // 外部为正
      }

      void main() {
        float d = triangle_distance(vUv, vec2(0.3), vec2(0.5, 0.7), vec2(0.7, 0.3));
        d = fract(20.0 * abs(d));
        gl_FragColor.rgb = vec3(d);
        // gl_FragColor.rgb = (smoothstep(0.45, 0.5, d) - smoothstep(0.5, 0.55, d)) * vec3(1.0);
        gl_FragColor.a = 1.0;
      }
    `;

    const canvas = document.querySelector('canvas');
    const renderer = new GlRenderer(canvas);
    const program = renderer.compileSync(fragment, vertex);
    renderer.useProgram(program);

    renderer.setMeshData([{
      positions: [
        [-1, -1],
        [-1, 1],
        [1, 1],
        [1, -1],
      ],
      attributes: {
        uv: [
          [0, 0],
          [0, 1],
          [1, 1],
          [1, 0],
        ],
      },
      cells: [[0, 1, 2], [2, 0, 3]],
    }]);

    renderer.render();
  </script>
</body>
</html>